The human parvovirus, adeno-associated virus (AAV), has been considered a potential vector in applications of gene therapy. Recombinant forms of the vector can be generated in which all viral open reading frames are deleted. We learned previously that purified recombinant AAV exhibits a poor rate of transduction in most applications. AAV transduction can be increased several logs in the presence of adenovirus, through expression of E1 and E4, which enhances the conversion of the AAV single-stranded genome to a non-integrated, transcriptionally active double-stranded intermediate. We, and others, discovered that purified recombinant AAV very efficiently transfers genes into differentiated fibers of skeletal muscle in vivo in the absence of adenovirus. Remarkably, there is little immune response to the product of an AAV transduced gene in the context of muscle. Recent data suggest that liver may have similar properties as muscle with respect to AAV transduction: intravenous infection of high- titer recombinant AAV yields moderately efficient, stable, and non- immunogenic gene transfer to murine liver. The goal of this project is to further explore the potential of AAV as a vector by evaluating its biology in the context of gene delivery to skeletal muscle and liver. Specific Aim #1 studies the mechanism or recombinant AAV transduction in skeletal muscle and liver, focusing on rate-limiting steps in transduction efficiency and molecular analyses of the vector genome. The immunology of recombinant AAV transduction in both skeletal muscle and liver will be evaluated in specific aim #2. This will include a detailed analysis of the mechanisms by which AAV evades immune detection of antigenic transgene products. Additional studies on humoral immune responses will be performed. Concurrent with the emergence of AAV as a potentially useful vector has been the concept of developing adenoviruses deleted of all open reading frames. We will evaluate the performance of these fully deleted adenoviruses in specific aim #3 as compared to the data generated with AAV. Project 1 provides a foundation of vector biology necessary for the disease-based projects 2 and 3.